The present invention relates to a method and apparatus for easily and quickly removing the inner conduit of a dual passage drill string used in drilling geothermal, gas, or oil wells, and, more particularly, to a system and method for removing an inner conduit which is used to deliver high pressure fluid to a drill bit in combined mechanical and jet drilling operations. Although the invention is described herein in connection with geothermal, gas, and oil well drilling, the principles and concepts disclosed apply equally to other forms of drilling and mining.
In oil and gas well drilling, the cost of equipment and labor is extremely high. In order to minimize the cost of this phase of oil and gas production, it is desirable to drill wells through earthen formations as rapidly as possible, commensurate with good drilling practices. However, in harder formations, which are more difficult to drill, there are a number of operating problems that tend to make the cost of such holes more expensive.
In order to increase the drilling rate of deep, hard-to-drill wells, new technology has been developed. For example, it has been shown in previous tests that high pressure fluid jets at 12,000 pounds per square inch (PSI) or more at the drill bit can significantly increase drilling rates. Previously, this high pressure fluid was delivered to the drill bit by pressurizing the entire drilling fluid stream (or "mud"), and conducting it to the drill bit through standard drilling pipe having improved seals at their connections. However, the costs associated with raising such a large volume of fluid to high pressures, together with the increased internal pressures on the seals and drill pipe connections, led to drill pipe failures, significant down time, and increased costs, even though drilling rates were higher. Thus, these attempts at jet drilling proved to be impractical.
Recently, Flow Industries, Inc., assignee of the present application, devised a dual conduit approach for successfully utilizing the principles of jet drilling. In this approach, described in more detail and claimed in copending application Ser. No. 661,368, U.S. Pat. No. 4,624,327, filed Oct. 16, 1984, and entitled "METHOD AND APPARATUS FOR COMBINED JET AND MECHANICAL DRILLING", which is hereby incorporated by reference, a high pressure liquid is conducted via an inner conduit to the drilling bit to provide either a jet assisted mechanical drill or a mechanically assisted jet drill. The drilling mud is delivered to the drill bit in the annulus between the inner conduit and the outer conduit or standard drill pipe. This approach overcomes the problems faced in previous jet drilling methods, although it does require a dual conduit drill string.
Dual conduit drill string presently available is manufactured so that the inner conduit is an integral part of the outer drill pipe, and is therefore unremovable with respect thereto. Thus. it is impossible to pull the inner conduit out of the outer drill pipe while the latter remains in the well. This situation is a serious disadvantage under certain conditions where servicing or repair of the drill pipe is necessary.
Occasionally, the drill pipe breaks. In order to reduce cost, as much of the drill string as possible is salvaged. Thus, the drill string must be cut off at the appropriate depth and removed. In order to accomplish this salvaging operation, certain tooling must be inserted down into the drill pipe. With the inner conduit in place, however, the salvage operation is severely hampered. On other occasions, the drill pipe may be stuck in the hole and cannot be moved either way. A small explosive charge may then be detonated in the hole in order to break the drill pipe at a threaded connection, again in order to save as much of the drill pipe as possible. At other times, articles become caught in the drill pipe and must be fished out. In all of these situations, the existence of the inner conduit in the drill pipe impedes and sometimes prevents the salvage or repair operation.
Thus, there is a need for a dual passage drill string in which the inner conduit can be removed independent of the drill pipe.